Why is it that some posts, when edited, leave a date/time stamp on the post saying that they were edited while others do not?

It depends on how long after you post that you make the edit. Sometimes if I make an edit to one of my posts within a couple minutes it doesn't show it. If I go back the next day it will._________________Sad state of affairs.

Ok. That's what I noticed as well - if I do it within a couple of minutes it doesn't seem to show. If I do it a long time after it does show. I didn't know if it was time related or related to something else such as which forum...